1. Field of the Invention
The present invention generally relates to bore casings which are driven through soils to form or help form structural supports or throughways. More specifically, the present invention relates to a reduced skin friction bore casing for reducing the frictional forces applied to the bore casing by soils and other foundation materials.
2. Description of the Related Art
Deep foundations are utilized to support structures such as, but not limited to, buildings and bridges where near surface in-place soils do not have adequate strength to support the anticipated structural loads. The present invention relates to bore casings generally utilized in the foundation construction industry, which may be driven, pushed, drilled, or otherwise forced into soils and then used to help construct piles or caissons. Quite often these piles are driven through soft compressible soils into hard underlying soils, partially weathered rock, or rock. The support of the piles is provided by these underlying hard materials.
Bore casings may be used for both temporary and permanent applications. Permanent applications refer to those instances where the bore casing is driven, pushed, or drilled into the soils, and the bore casing is left in place as either a portion of the support structure, or possibly a throughway as would be used to house cabling, utility piping, etc. Temporary applications refer to those instances where the bore casing is driven, pushed, or drilled into the soil, the soil present inside the bore casing is excavated, and fluid concrete or another suitable material is introduced to the interior of the bore casing for forming a pile or caisson. Prior to the concrete completely solidifying, the bore casing is removed, leaving the concrete pile behind as a structural element. Temporary and permanent applications for bore casings pose unique sets of problems.
Typically, the bore casings used for permanent applications are of relatively small diameter although it may be desirable to leave a large diameter bore casing in the soil as a portion of a support structure as well. The bore casing is usually driven through compressible soils to a sufficient depth so that the downwardly driven end of the bore casing penetrates harder underlying materials to a specified depth or until refusal is achieved. Typically, significant support of the piles is provided by these underlying harder materials.
When used in permanent applications, the bore casing will be subject to a downward frictional force on the bore casing that is a function of the horizontal stress applied to the bore casing by the soil and the coefficient of friction of the bore casing""s exterior surface relative to the soil. This downward force can result in failure of a structural support due to an unexpected downward movement of the bore casing enclosed pile or caisson, or increase the time required to install the pile or caisson.
For many years, departments of transportation, structural engineers and geotechnical engineers have struggled with the problem of how to reduce downward frictional forces imposed upon piles. Many costly measures have been implemented to address this problem, including: delaying construction to allow underlying soils to consolidate and settle; utilizing support structures designed for an increased load capacity; and pre-drilling a hole and re-filling the hole with a lubricating material or pea gravel. These methods result in considerable increased construction costs.
In cases calling for delaying construction to allow underlying soils to settle, both workers and machinery can remain idle for extended periods of time, thereby driving up costs. Lubricants and other substances placed in pre-drilled holes lend themselves to environmental concerns.
In cases in which the anticipated structural load on the support structure is increased to account for a downward frictional force, a higher capacity support structure is required. This requires driving the bore casing farther into the harder consistency soils, thereby requiring an increase in bore casing length and an increase in the capacity of the driving hammer capable of driving a bore casing to a higher criteria. In some cases, these requirements increase the cost of constructing the support structure and the length of time for support structure installation and may require an increased cross sectional area of the bore casing to allow for the higher capacity.
When used for temporary applications, other frictional forces come into play. Typically,. the bore casing will be driven, pushed, or inserted in a pre-drilled hole. Once in place, unless the hole has been pre-drilled, the soil present in the internal space of the bore casing is removed through drilling. After a void has been created to the desired depth, fluid concrete or another suitable material is poured into the void. Prior to the final solidification of the concrete to form the support structure, the bore casing is removed for use in the formation of other support structures. As would be expected, reduction of insertion and extraction forces will reduce the time and effort required to create the support structure.
Therefore, there is a need to reduce the friction forces between the bore casing and the soils and construction materials contacting both the interior and exterior surfaces of the bore casing during insertion. Also of interest is reducing the upward friction forces exerted by the soil on the inner surface of the bore casing during excavation of the internal volume of the bore casing. Importantly, reduction of downward frictional forces during extraction is desired. This is the point in the process when the greatest forces will probably be encountered. It is possible that the downward frictional forces exerted by the soil on the exterior of the bore casing in combination with the downward frictional forces exerted by the hardening concrete on the inner surface of the bore casing can make removal of the bore casing by standard methods problematic. In this case, the bore casing is either left in position or extracted using great time and effort, and potentially severely damaging the support structure as a result. Even if the bore casing is eventually removed leaving the support structure in place, excessive friction with the concrete support structure could cause the support structure to be partially raised with the bore casing. This can lead to voids in the support structure as well as the thinning (or necking) of the support structure and weaken it greatly. If the bore casing is left in the soil, this could cause later structural problems as the frictional calculations for the support will have been completed using a frictional coefficient of concrete, rather than that of the bore casing.
Therefore, there is a need for improved bore casings which address these and other shortcomings of the prior art.
Briefly described, the present invention relates to a reduced skin friction bore casing which reduces the frictional forces applied to the pile by soils and construction materials encountered during insertion and extraction operations, as well as by the settling of compressible soils surrounding the bore casing after the bore casing has been driven into the soil. A preferred embodiment of the reduced skin friction bore casing is particularly suited for driving into, and removal from, soils. Preferably the bore casing is composed of a material such as steel, and includes both an exterior and interior surface that are substantially concentric about a longitudinal axis. A friction reduction coating is applied and adhered to portions of the exterior and/or interior surfaces with enough strength and abrasion resistance to withstand abrasion forces applied to the bore casing during driving operations into the soils as well as extraction when required.
In accordance with another aspect of the present invention, a preferred method includes the steps of: (1) providing a bore casing of constant size and shape along a longitudinal axis, having an interior surface and an exterior surface; (2) adhering a friction reduction coating to the exterior surface with enough strength and abrasion resistance to withstand the abrasion forces applied to the pile during insertion operations into the soil; and (3) and inserting the bore casing into the soil. In those applications where the bore casing is not to remain in the soil as a part of a support structure, the method can further include the steps of: (1) adhering a friction reduction coating to the interior surface with enough strength and abrasion resistance to withstand the abrasion forces applied to the pile or caisson during insertion and extraction operations; (2) excavating the soil from the inner volume of the bore casing; (3) filling the inner volume of the bore casing with concrete; and (4) extracting the bore casing from the soil.
Other objects, features and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.